Despite the critical importance of monitoring and measuring the relative state-of-charge (SOC) of an electrolyte entering or exiting an electrochemical cell or stack in real time, the prior art in energy storage and utilization does not teach robust methods or apparatuses for doing so in liquid systems. All previous attempts to conduct such measurements appear to be subject to fouling, drifting, or otherwise becoming unreliable over time. Furthermore, previous attempts measure and/or indicate the SOC of the battery (i.e., both positive and negative sides) rather than the SOC of an individual electrochemical half-cell (i.e., positive or negative side). The present inventions are directed to addressing at least one of these deficiencies.